What to Know Before a Colonoscopy – Preparation
In this article
The success of a colonoscopy largely depends on how clean the bowel is . In a poorly cleaned bowel, small polyps may be missed, the procedure may take longer or need repeating. For most people undergoing colonoscopy, preparation seems harder than the procedure itself. Yet current guidelines have made preparation much simpler: rather than days of dietary restrictions, most people need only one day of dietary restriction, and instead of 4 liters, 2 liters or less of fluid and medication taken in two parts. This article explains colonoscopy preparation step by step using current European and United States guideline recommendations, and separately evaluates the “two evening doses of soda” approach commonly used in Türkiye.
- Diet: For most people, restriction only on the day before is sufficient: light, low-fiber breakfast and lunch, followed by clear liquids. People with risk factors such as constipation or diabetes, or according to their center's instructions, may need a low-fiber diet for 2-3 days.
- Medication in two parts: The most effective method is to take half the medication the evening before and half on the procedure morning (split dose). Start the second dose 4-6 hours before the procedure and finish at least 2 hours beforehand.
- “Two evening doses of soda”: Some centers in Türkiye administer two doses of sodium phosphate the evening before, for example at 20:30 and 22:30. This is practical but unsuitable for everyone because of kidney risks; without a morning dose, cleansing may be less effective than with split dosing.
- What Are Clear Liquids? Water, weak tea, pulp-free apple juice, fat-free broth and light-colored sports drinks. Ayran, milk, juice with pulp, and red or purple drinks are not included.
- The final 2 hours: For sedation safety, take nothing by mouth during the 2 hours before the procedure.
- Medications: Consult your doctor in advance about blood thinners, diabetes medications (particularly SGLT2 inhibitors and insulin), GLP-1 medications and iron tablets.
- The 2025 United States Multi-Society Task Force and current European Society of Gastrointestinal Endoscopy (ESGE) bowel preparation recommendations have been added.
- The previous recommendation of “5 days of low-fiber diet + 48 hours of soft diet” has been corrected to “only the preceding day for most people,” according to current guidelines.
- Preparation medications used in Türkiye (sodium phosphate, sodium picosulfate, senna) and the benefits and risks of “two evening doses of soda” have been added.
- The clear-liquid list has been corrected (ayran and sour cherry juice removed); medication management, assessing adequate preparation and when to contact a doctor have been added.
Why Is Preparation So Important?
During colonoscopy, the doctor examines the inside of the large bowel with a camera, millimeter by millimeter. Stool and cloudy liquid remaining on the bowel wall can hide particularly small and flat polyps. We explain what colonoscopy is and how it is performed in our article What Is a Colonoscopy? .
According to the 2019 ESGE guideline, bowel preparation is inadequate in 18-35% of patients . Guidelines aim for adequate preparation in an endoscopy unit in at least 90% of procedures. If preparation is inadequate, repeat colonoscopy is recommended within one year ; inadequate preparation may therefore mean repeating the same difficult process. The likelihood of poor preparation increases with older age, constipation, diabetes, cirrhosis, Parkinson disease, dementia, a history of stroke, opioid painkillers and some antidepressants. If any apply to you, your doctor may plan a more intensive preparation.
Figure 1 · Colonoscopy Preparation Timeline
An example schedule for a morning appointment using split dosing.
How Is the Preparation Medication Taken? Different Approaches
The medications used for bowel cleansing and their administration times vary between centers. This variation can be confusing, but every approach has the same goal: a clean bowel during the procedure. The basic guideline recommendations are:
| Approach | How Is It Used? | Position in Guidelines |
|---|---|---|
| Split dose | Half the medication the evening before and half on the procedure morning; start the second dose 4-6 hours before and finish at least 2 hours beforehand | recommended First choice for all patients |
| Same day | All medication on the procedure morning | acceptable For afternoon appointments; less effective for morning appointments |
| Previous day only | All medication the day before, for example two evening doses | less effective Less successful cleansing than split dosing |
| Low volume (≤2 L) | 2 L or less of solution + additional clear liquids | preferred Similar cleansing to 4 L, better tolerance |
| Intensified preparation | 2-3 days of low-fiber diet, higher volume and/or additional laxative | For patients at high risk of inadequate preparation |
According to the ESGE analysis, split dosing increases the probability of “excellent or good” cleansing compared with taking all medication the day before by approximately 2,5-fold . The reason is simple: secretions from the small bowel accumulate in the beginning of the large bowel overnight. The second dose in the morning clears this new buildup too. The 2025 United States Task Force recommendations also recommend split dosing for all patients and prefer 2-liter regimens to 4-liter regimens.
“Two Evening Doses of Soda”: Who Is It Suitable For?
Some centers in Türkiye administer two doses on the evening before the procedure, for example at 20:30 and 22:30 of sodium phosphate solution (commonly called “soda,” marketed in Türkiye as Fleet Fosfa-soda); an enema may be given in the morning if needed. This approach has attractive features: a small medication volume (45 mL per dose, diluted with half a glass of water), relatively tolerable taste and completion of preparation in the evening.
However, it has two important limitations:
- Kidney risk: Sodium phosphate has been associated with acute phosphate nephropathy , which can rarely cause permanent kidney damage. In December 2008, the United States Food and Drug Administration (FDA) recommended prescription-only use of these products for bowel cleansing; the manufacturer withdrew over-the-counter products from the United States market, and a boxed warning was added to tablet formulations. ESGE does not recommend routine useof sodium phosphate and states that it should not be used in kidney disease (GFR below 60), heart failure, inability to drink enough fluids, electrolyte abnormalities, abdominal fluid accumulation (ascites), pregnancy or people under 18.
- Not split dosing: When both doses are taken the same evening, new secretions accumulate in the bowel before morning. Cleansing can therefore be less effective than split dosing, particularly for morning appointments. Notably, the Turkish product instructions also describe a longer interval rather than two doses the same evening: for a morning appointment, the first dose at 07:00 the preceding morning and the second in the evening; for an afternoon appointment, the first dose at 19:00 the preceding evening and the second at 07:00 on the procedure morning.
This method can be used under medical supervision, strictly following instructions, in young people without kidney or heart problems, not taking regular medication and able to drink plenty of fluids. The key is for your doctor to know your risks and select instructions for you. Do not take preparation medication on your own using instructions found online.
Figure 2 · Is Sodium Phosphate (“Soda”) Suitable for You?
An alternative preparation medication should be preferred in people with risk factors.
Preparation Medications Used in Türkiye
Fleet Fosfa-soda, two 45 mL doses. Low volume, but not used in kidney disease, heart failure or dehydration; caution is needed in older adults and with some blood pressure medications. Drink at least one full glass of cold water and plenty of clear liquids after each dose.
Citrafleet, two sachets. Dissolve each sachet in approximately 150 mL of water; leave at least 5 hours between sachets and drink 1,5-2 liters of clear liquids after each. Not used in severe kidney failure or heart failure.
Centers also use PEG-based solutions, senna preparations (such as X-M) and enemas. Each medication is taken differently; follow only the written instructions given to you.
Diet: What to Eat and When
Previously, a low-fiber diet from 5 days before colonoscopy and a soft diet from 48 hours before were recommended. Current guidelines simplify this: ESGE recommends limiting a low-fiber diet to the day before the procedure ; it cleans as effectively as a clear-liquid-only diet and is better tolerated. The United States Task Force likewise recommends restricting the diet only on the preceding day, with clear liquids or low-fiber foods for breakfast and lunch. Those at high risk of inadequate preparation, such as chronic constipation, diabetes or inadequate previous preparation, may start the diet 2-3 days before.
Many centers in Türkiye still recommend 3-5 days of low-fiber diet. This is not harmful; it is simply unnecessary for most people. Follow your center's instructions.
| Period | Foods / Drinks Allowed | Avoid |
|---|---|---|
| Low-fiber diet (breakfast and lunch the preceding day; from 2-3 days before if needed) |
White bread, pasta, rice, eggs, lean chicken and fish, peeled well-cooked vegetables, peeled seedless fruit, plain yogurt | Whole grains, wholemeal bread, nuts, seeds, corn, popcorn, raw vegetables and salad, fruit with skins and seeds, dried legumes, fried foods |
| Clear liquids (from the preceding afternoon) |
Water, weak tea, black coffee (no milk), pulp-free apple or white grape juice, strained fat-free beef or chicken broth, light-colored sports drinks, plain mineral water, light-colored sodas | Ayran, milk and dairy products, juices with pulp, unstrained soups, red and purple drinks (sour cherry juice, pomegranate juice, red sports drinks), alcohol |
Red and purple drinks can resemble blood or lesions on the bowel surface. If you have diabetes, choose sugar-free clear liquids instead of sugary sports drinks and check your blood glucose more often.
One Week Before: Supplies and Medications
- Preparation medication: Obtain the prescribed medication and written instructions in advance; read the instructions one week before, not the day before.
- Clear liquids: Expect to drink a total of 2-4 liters of clear liquids; purchase light-colored drinks in advance.
- Skin care: Alcohol-free wipes and zinc-containing diaper rash cream for the anal area reduce irritation from frequent bowel movements.
- Straw and lemon: To reduce the medication's taste.
- Companion: Arrange someone to take you home after a procedure involving sedation.
Discuss all medications with your doctor before preparation. The following are particularly important:
| Medication | General Approach (Your Doctor Makes the Final Decision) |
|---|---|
| Aspirin | Usually continued. |
| Blood thinners (clopidogrel, warfarin, apixaban, rivaroxaban, etc.) | Diagnostic colonoscopy alone, including biopsy, is considered low risk, but planning is often needed because polyps might be removed. According to the European and British guideline, if polypectomy is planned, drugs such as clopidogrel may be stopped 7 days and warfarin 5 days beforehand; timing for newer anticoagulants varies by medication and kidney function. Do not stop them yourself if you have a heart valve, stent or history of clots. |
| Iron medications | Many centers request stopping them several days before, generally 5-7 days, because they darken stool and complicate cleansing. We discuss iron medications in this article . |
| SGLT2 inhibitors (dapagliflozin, empagliflozin, etc.) | Because fasting and dehydration increase ketoacidosis risk, the FDA recommends stopping them at least 3 days before surgery; the same approach is generally used for colonoscopy preparation. |
| Insulin and other diabetes medications | Doses may need reducing because of low blood glucose risk during the clear-liquid period. |
| GLP-1 medications (semaglutide, tirzepatide, etc.) | According to the 2024 multi-society recommendations, most patients can continue treatment; those on high doses should follow a liquid diet for the preceding 24 hours. The procedure may be postponed during dose escalation or if nausea and vomiting occur. |
| Diuretics, ACE inhibitor or ARB blood pressure medications, NSAID painkillers | They increase kidney risk with sodium phosphate and are considered when selecting preparation medication. |
| Other daily medications | Most may be taken with a little water on the procedure morning, at least 2 hours beforehand; absorption of oral medications taken immediately after bowel preparation medication may be reduced. |
We also discuss the general approach to blood thinners in our article on stomach biopsy .
Preparation Evening and Morning: Practical Tips
- Prepare the solution in advance and chill it in the refrigerator; drinking it cold softens the taste.
- Use a straw, placing it near the back of the tongue; sucking a lemon slice or chewing sugar-free gum between drinks masks the taste.
- Drink regularly, taking a few minutes per glass, approximately one glass every 10-15 minutes; do not try to drink it all at once.
- If nausea occurs, pause for 30-60 minutes and then resume more slowly.
- Some centers add simethicone to the preparation liquid to reduce foam and improve visibility; ESGE considers this reasonable. Do not add it yourself unless your doctor recommends it.
- Moving around at home may encourage bowel movements; stay near a toilet.
Is Your Preparation Adequate?
Figure 3 · Is Your Preparation Adequate?
Stool color and clarity after the final doses are useful indicators.
- Severe abdominal pain, a rigid abdomen or vomiting that prevents you from drinking the medication.
- Dizziness, feeling faint, palpitations or very little urine (signs of dehydration).
- No bowel movement within 6 hours of taking preparation medication.
- Very low or high blood glucose if you have diabetes.
- Stool remaining dark with pieces after the final doses.
2 Hours Before: Why Must You Stop Drinking?
Colonoscopy is usually performed under sedation (sleep medication given intravenously). Swallowing and coughing reflexes weaken during sedation; fluid in the stomach can enter the lungs. According to the American Society of Anesthesiologists, clear liquids may be consumed until 2 hours before the procedure, then nothing should be taken by mouth. The previous explanation, “for comfort afterward,” was incorrect; the actual reason is sedation safety. Some centers request no drinks after midnight; follow your center's instructions.
If you received sedation, do not drive or make important decisions that day; return home with a companion.
Afterward: When Is the Next Colonoscopy?
For an average-risk person with a normal, complete colonoscopy and adequate preparation, the next screening is generally recommended 10 years later . If polyps were removed, the interval is shortened according to their type, number and size. If preparation was inadequate, repeat the procedure within 1 year. We discuss screening intervals after normal colonoscopy in this article, and polyps in our article What Is a Polyp? .
- The population-based screening program recommends, for average-risk people aged 50-70 , a fecal occult blood test every two years and colonoscopy every 10 years; services are available at KETEM cancer screening centers, community health centers and family health centers.
- A positive fecal occult blood test requires colonoscopy; we explain what happens next in this article .
- For people unable or unwilling to undergo colonoscopy, we compiled other options in 7 Alternatives to Colonoscopy . We also discuss the reduction of the screening age to 45 in United States guidelines in this article .
- Preparation medication and instructions vary between centers; even the same institution may give different instructions according to appointment time.
Frequently Asked Questions
- My Center Prescribed Two Evening Doses of “Soda.” Is It Safe?
- It can be used as directed by your doctor if you have no kidney or heart problems, are not taking diuretics or certain blood pressure medications, and can drink plenty of fluids. If any apply or you are older, tell your center and request an alternative preparation medication. Always drink the recommended water and clear liquids after each dose.
- Must I Follow a Diet for 5 Days?
- Current guidelines say the preceding day's diet is enough for most people. With risk factors such as constipation, diabetes or inadequate previous preparation, a low-fiber diet for 2-3 days may be recommended. Follow your center's instructions if they differ.
- Can I Drink Ayran or Soup?
- Strained soup without vegetable pieces and plain yogurt may be allowed during the low-fiber diet. Once the clear-liquid period begins, do not drink ayran, milk or unstrained soup; these can coat the bowel surface.
- Can I Drink Water After Midnight?
- Anesthesiology recommendations allow clear liquids until 2 hours before the procedure. However, some centers require longer fasting; follow the instructions of the team administering sedation.
- I Took the Medication but Have Not Had a Bowel Movement. What Should I Do?
- The effect may take several hours in some people. Moving around and continuing to drink clear liquids may help. Call your center if there is no bowel movement within 6 hours of the first dose, or if severe abdominal pain, bloating or vomiting occurs.
- My Anal Area Is Irritated During Preparation. What Can I Do?
- Use alcohol-free wipes instead of toilet paper and apply zinc-containing diaper rash cream before and throughout preparation. Hemorrhoids can increase irritation; a warm sitz bath may help.
- What Happens if Preparation Is Inadequate?
- The doctor may be unable to see polyps reliably. Guidelines recommend repeating colonoscopy within 1 year after inadequate preparation. Some centers may attempt additional cleansing and completion the same or following day.
- I Have Diabetes. What Should I Consider?
- Diabetes increases inadequate preparation risk, so your doctor may recommend a 2-3-day diet. Because low blood glucose can occur during the clear-liquid period, plan insulin and other medication doses with your doctor in advance; you may need to stop SGLT2 inhibitors several days before.
Glossary
- Split dose
- Taking half the preparation medication the evening before and half on the procedure morning.
- Low-fiber (low-residue) diet
- A diet leaving little bowel residue, without bran, seeds, skins or raw vegetables.
- Clear liquid
- A see-through liquid containing neither pulp nor milk.
- Sodium phosphate
- A low-volume preparation medication that draws water into the bowel and causes diarrhea; commonly called “soda.”
- PEG (polyethylene glycol)
- A nonabsorbed solution that cleanses by retaining water in the bowel; available in 1-4-liter formulations.
- Sodium picosulfate
- A low-volume preparation medication that stimulates bowel movements and is used with magnesium citrate.
- Acute phosphate nephropathy
- Kidney damage caused by phosphate accumulation, occurring rarely after sodium phosphate use.
- Sedation
- Light or deep sleep induced by intravenous medication during the procedure.
- Simethicone
- Medication reducing foam and gas in the bowel.
- Screening colonoscopy
- Colonoscopy in people without symptoms to detect cancer and polyps early.
DROZDOGAN Academy Commentary
Colonoscopy is one of the few tests that can both detect colorectal cancer early and prevent it by removing polyps. Many avoid it because they fear preparation rather than the procedure itself.
⚡ Why It Matters
Inadequate preparation reduces colonoscopy's preventive value and requires repeating it. Easier preparation is among the simplest steps that might improve screening participation.
✅ Strengths
Current guidelines have substantially simplified preparation: a one-day diet, low-volume medication and split dosing. These changes reduce patient burden without compromising cleansing quality.
⚠️ Limitations
Centers in Türkiye differ considerably in diet duration, medication and timing. Sodium phosphate is widely used because it is easy to drink, but European guidelines do not recommend routine use because of kidney risks. We could not find an up-to-date national preparation guideline for Türkiye.
🩺 What to Do in Practice
- Read your instructions one week beforehand ; ask your center about anything unclear.
- Report kidney or heart disease and medications before preparation medication is selected to your care team.
- If possible, prefer instructions using split dosing ; discuss this with your doctor if booking a morning appointment.
- Keep clear liquids truly clear: no ayran, milk, or red-purple drinks.
- Take nothing by mouth in the final 2 hours before the procedure, and return home with a companion.
❓ Open Questions
How do preparation regimens affect cleansing quality in Türkiye? How often do kidney problems occur with sodium phosphate? Can tablets, mobile app reminders and individualized regimens reduce inadequate preparation below 10%?
Our Brief Verdict: Colonoscopy preparation no longer needs to be as difficult. For most people, a one-day diet and low-volume split-dose medication suffice. “Two evening doses of soda” are practical but unsuitable for everyone; selection should consider kidney and heart health and medications.
- This is the September 2026 update of our article “What to Know Before a Colonoscopy – Preparation.” It draws on the European Society of Gastrointestinal Endoscopy (ESGE) 2019 guideline, United States Multi-Society Task Force 2025 recommendations, FDA, American Society of Anesthesiologists, British and European gastroenterology society antithrombotic guidance, the 2024 multi-society GLP-1 recommendations and Turkish medication leaflets.
- Corrections to the Previous Version: “5 days of low-fiber diet + 48 hours of soft diet” was changed to “the preceding day for most, 2-3 days with risk factors” according to current guidelines. Ayran and sour cherry juice were removed from clear liquids. “Half the dose in the evening, half 6 hours before” was clarified as “start the second dose 4-6 hours before and finish at least 2 hours beforehand.” The reason for “nothing to drink 2 hours before, for comfort afterward” was corrected to sedation safety. Sodium phosphate, sodium picosulfate and other medications, medication management, adequacy assessment and warning signs were added.
- Information From Secondary Sources: The United States Task Force 2025 recommendations were taken from abstracts and society summaries because the full text was unavailable; antithrombotic recommendations from a guideline summary; the 2008 FDA sodium phosphate decision from a review; and Türkiye's screening program from a family health center's program summary.
- Not Verified: A national bowel preparation guideline for Türkiye; PEG-based products marketed in Türkiye; a guideline recommendation on how many days before to stop iron (reported as center practice); and a standard insulin dose-adjustment recommendation.
- Based on General Knowledge: The split-dose mechanism (secretions accumulating overnight), red-purple drinks impairing visibility, stool color as an indicator, drinking tips (chilling, straw, regular intervals), skin care and not driving afterward are presented as established practice.
- The classification in the guideline comparison table, Figure 2 decision flow and Academy commentary are our own.
- Diagrams derive from verified statements in the text and contain no new data.
- This article is not personal medical advice. Take preparation medication at the times specified in your procedure center's written instructions.
Sources
- Hassan C et al. Bowel preparation for colonoscopy: European Society of Gastrointestinal Endoscopy (ESGE) Guideline – Update 2019. Endoscopy. 2019;51:775-794.
- Jacobson BC et al. Optimizing bowel preparation quality for colonoscopy: consensus recommendations by the US Multi-Society Task Force on Colorectal Cancer. 2025 (Gastroenterology, Gastrointest Endosc, Am J Gastroenterol).
- Cleveland Clinic Journal of Medicine. Review of the United States Task Force 2025 bowel preparation recommendations. 2026;93(3):169.
- American College of Gastroenterology. Evidence-based GI: US Multi-Society Task Force bowel preparation recommendations. April 2025.
- OsmoPrep (sodium phosphate) United States prescribing information, boxed warning. FDA, 2018.
- Cleveland Clinic Journal of Medicine. Bowel preparation for colonoscopy (sodium phosphate and clear liquids). 2010;77(5):317.
- American Society of Anesthesiologists. Practice guidelines for preoperative fasting. 2017.
- Veitch AM et al. Endoscopy in patients on antiplatelet or anticoagulant therapy: BSG and ESGE guideline update. 2021.
- American Society of Anesthesiologists et al. Multi-society clinical practice guidance for the safe use of GLP-1 receptor agonists in the perioperative period. October 2024.
- FDA. Stopping SGLT2 inhibitors before surgery (Healio report, March 2020).
- Fleet Fosfa-soda 45 mL oral laxative, patient leaflet (Türkiye).
- Citrafleet, patient leaflet (Türkiye, Recordati).
- Republic of Türkiye Ministry of Health national colorectal cancer screening program standards (family health center summary).
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